Two-tone radio telegraph transmission bias elimination



Sept. 16, 1947. A. .-MATTE 2,427,363

TWO TONE RADIO TELEGRAPH TRANSMISSION BIAS 'ELIMINATION Filed-Des. 51, 1943 Patented Sept. 16, 1947 TWO-TONE RADIO TELEGRAPH TRANS- MISSION BIAS ELIMINATION Andrew L. Matte, Summit, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application December 31, 1943, Serial No. 516,389

3 Claims.

This invention relates to telegraph systems and more particularly to the elimination of bias in telegraph signals.

An object of this invention is the elimination of bias in telegraph signals.

The invention herein is exemplified, in the present embodiment of the invention in a radio telegraph system, but the invention may be widely applied in other telegraph systems.

In certain types of telegraph signal transmission, marking and spacing signal elements are of two different current conditions.` These current conditions may be current and no-current, positive and'negative current or current of two different frequencies or magnitudes, etc.

In the present embodiment, the invention is applied to a 2-tone or frequency modulation telegraph system in which signals are formed by transmitting successively brief pulses of one or the other of two frequencies. We shall refer to these two conditions hereinafter as marking and spacing respectively.

The telegraph signal code considered herein consists of various combinationsv of successive pulses of these two frequencies, each for a definite length of time. `It follows that if for any reason the duration of one ofthe two conditions becomes distorted beyond a certain point, 'a character other than the Vdesired character will be produced. If this distortion is systematic, causing a lengtheningof all marks or a lengthening of all spaces,

and furthermore ifV its character is vsuch that an interchange of the marking and spacing functions ofV the current controlling the transmitter causes the distortion previously associated with the marks to become associated with the spaces and vice versa, this distortion is referred to technicallyas bias. t f

In considering the effect of `bias in producing incorrect signals, it is pointed'out that 'bias constitutes only one kind of distortion and is algebraically additive to allA other kinds of distortion. In the present embodiment thisbias is caused .by the travelftime of an armature operating between` opposed contacts;

For illustrative purposes, it will be assumed that it is desired to transmit a uniform series of telegraphfdots, i. e., signals in which marking and spacing inter- ,vals .are equal.r The armature is in engagement with each of its opposed contacts for equal intervals, but th-e'transmitting circuit controlled ,-by the armature is so arranged that the travel time'of the armature in each direction between contacts is' added to the'duration of a particular ,one-ofthe two signaling conditions. The result is that one of the two signaling conditions is of longer duration than the other by an interval equal to twice the travel time of the armature, the travel time in each direction being substantially equal. This interval is appreciable. The shorter the interval of contact engagement, the larger the ratio of bias to signal length, assuming a xed travel time. Bias tends to limit the speed of operation of telegraph transmitters. As the speed is increased, bias seriously aiects reception and in transmission at high speeds often becomes a limiting factor.

Various methods of bias elimination are known in the art. None of the presently known methods would be effective, however, to eliminate bias in the radio telegraph system to which the bias elimination of the present invention is applied.

'I'he invention may be understood from reference to the attached drawings, in which:

Fig. 1 shows a well-known radio telegraph signal transmitter which does not include the bias elimination arrangement of the present invention;

Arelay |02 is actuated to engage the opposed contacts |04 and |05. Y When the armature |03is in engagement with its spacing contact |05, aV circuit may be traced from ground I 06 throughepositive battery |01 and resistance |08 to junction point |09, where parallel branches are formed. One branch eX- tends from junction point |09 through contact |05 and the armature of relay |02 to ground H6. point |09 through the right-hand portion 0 of resistance to junction point ||2 where parallel branches are formed. One parallel branch connects to the grid of a well-known reactance tube H3. The other parallel branch extends through the left-hand portion ||2 of resistance I|| to ground ||5. With the circuit Fig. 2 shows the radio telegraph signal trans- Another vbranch extends from junction in this condition with direct ground H6 connected to junction point 03, the grid of reactance tube H3 is at ground potential. In response to this, reactance tube H3 Will cause a frequency fi to be emitted by the transmitter by any one of several well-known methods. The output circuit of tube .I I3 is ultimately connected to la transmitting antenna, not shown. Reference is made to Patent 2,258,470 to Karl Rath, October 7, 1941, and to an article by Travis in the I. R .E. proceedings of October .193,5 for vexplanations of the manner in which electronic reactance devices control tuning or oscillating frequency, and such disclosures constitute .'a ipart `of this present disclosure.

As long as armature |03 Aremains in Aengagement with spacing contact i, vthe-transmission of current of frequency fl will continue. The instant that the armature l0? disengages from contact i052, however, ground H6 will be disconnected from juncticn point |03 and from -the fgr'd of tube Al I3. .-'A positive `potential -equal to .the potential impressed vbetween point HZ 4and bground Il 5 will be impressed between `.the grid @of tube l I3 and f ground.

In response to this, 4a currentof frequency :f2 Williiow in the output -circuit'of reactance ltube H3. ,This condition will persist during the interval while armature 02 is traveling fromcon- "tact .1.05 to contact i045, plus the -intervlal 4While `armature i352 remains infengagementiwith contact |04, plus the interval while the `armature 1362115 traveling :from Contact l 0.13 back fto contact l-GE.' -iIh-e current of frequency IJl will flow only during the Vinterval while the .armature 162 Vis in engagement with contact [03. Thus -With -the arrangement per Fig. l the i duration 4of the fmark- -ing signal element will exceed the `duration of the spacing .signal .elem-.ent .by the sum of the armature :travel times ii-n each direction.

lThis eisindicated by the :curve b Ain Fig. 4 where frequency is .plotted vagainst time. YIt `is assumed that unbiased signals are received through conductor l0! as 'indicated rin .curve-a in Fig. e. The dunationnf :the marking signal element lul .corresponding Vto frequency f2 yis longer v.than the duration of the spacing signal element '402 `corresponding to .frequency jl .by an amount equal `to '2t lor 4twice the armature 'travel time.

Refer now `to Fig. 2. In^Fig. '2, arresistance 22H has been added fto the yinput circuit of the .re-

actance tube 2I3. When armature 120.3 .of :relay 23.2 is in .engagement with spacing contact 235, ground 21.6 "is connected to junction point 2.00.

Thegridof the vtube 21.3 is .at ground potential.

A frequency indicated by f3 in Fig. 5 is `transmitted. When contact 264 is engaged, resistance 2li vis short-circuited by 4a path from point :269 through armature 203 and contact 204 rto point 2l8. the grid of tube 213 and ground. The frequency transmitted by reactance tube `2|3 is indicated by fd in Fig, .5. When .the .armature 203 is traveling .between contacts '205 .and 2 0.5, in `either direction, resistance 2| 1 is connected inthe input circuit of tube 213. A positive potential smaller in value Ythan for the marking condition will be impressed between the grid of tube 2|3 and ground .and a frequency fm intermediate f3 and f4 will be transmitted, as indicated in Fig. 5, .during the travel interval in each direction.

t Assuming unbiased Asignals received through conductor 204, as indicated by curve c in Fig. .5, .the signals transmitted by the tube 2|3 Will be .as indicated in curve d in Fig. 5.

A positive potential is impressed between v Fig. 2. yceived ysignal impulses iare conducted through The manner in which the receiver per Fig. 3 functions to translate the signals per curve d Fig. 5, transmitted by the apparatus per Fig. 2 into unbiased signals will now be described.

Refer to Fig. 3. Fig. 3 shows a receiving device arranged to receive the signals transmitted by From the receiving antennae the reconductor 30l. Signals of frequency f3 and fil A:are passed through band-pass filters 302 and 303, -respectively Signals of frequency fm are sup- .,pessed '..by :both filters.

Signals of frequency f3 and fd are impressed on detectors 305 and 365 @and :corresponding .rectified impulses are im- ;pressed'onlthe middle and top windings of polar relay 366. -If it is assumed that there is no .fortuitous distortion present due to static and that there is no drift in frequencies, the bottom Winding lof relay 306 is not required. The armature of relay 306 upon being operated to one Contact Ior the other will -remain in :engagement Vwith that contact runtil the following marking or spacing impulse is received. `Thus the Vtravel time of the armature of .relay'20f2in one direction will be added say -to 'the Vmarking v interval and the vtravel-time in the-opposite direction'will be added to the spacing inter-.val .eliminating bias.

In cases where there is fortuitous distortionfdue to static or where there is adrift in :frequency -so that signals of the intermediate frequency fmy are not sufficiently attenuated yby `filters 302 and 303,the relay .306may include a'windingfarranged -as isits tbottornwinding `in Fig. 3.

When the Yarmature --of v`relay 3,06 engages lits `Contact VSEV! a circuit may y-be traced from ground through negative battery 310, .contact 301, 're- `sistance 3 09 and .thebottom windingof-:relay '306 back to ground. The effect 'of 1current'flowing through this circuit tends to hold the varmature of relay 306 in engagement with ycontact :30.1. When the armature of relay 30.6 :engages tcontact 308 4a circuit may be'traced fromground through positive battery 3H, 'contact 30.8, resistance .3.09, and the bottom winding .of relay 3,06 back to ground. The effect of -curren't flowing in this pathtends to maintain the varmature of :relay 30.6

in engagement with contact .308.. vThe lcurrents flowing through `the `bottom iwinding Afof yrelay 4'30.6 may be adjusted so that ythey are effective to maintain the armature of relay 306 in .engagement with 'the contact 'to which Vit was vlast operated against the effect `of such current of the intermediate frequency Afm as may be `passed by the band-pass filters 'as :wellas against the effect of the usual static.

In curve e in Fig. 6, f3, f4 and fmare plotted against attenuation for spacing and :marking Sfilters 302 and 303, respectively.

What is claimed 'ist 1. In a telegraph system, a telegraph transmitter, a reactance tube in said transmitter, a grid in said tube, a keying device comprising an armature controlling said itube, a first means, comprising a resistance circuit 'of a 'first magnitude connected to said grid, responsive to the Yoperation of said keying device for generating a marking signal of a first frequency when lsaid armature is in a first position, a second means, comprising a grounding `circuit for said grid, responsive to the operation of said :keyingfdevice for generating a spacing signal of a second frequency, different from said first frequency, when said armature is in a second position, a third means, comprising a resistance circuit of a secondmagnitude, different from said first magnitude, connected to said grid, responsive to the operation of said keying device, for generating an impulse of a third frequency, diiferent from said first and from said second frequency, continuously throughout the interval while said armature is in transit between said first and said second positions, telegraph signal receiving means responsive to the reception of signals of said rst and said second frequency for indicating a marking signal and a spacing signal respectively, said receiving means including also signal transition bias elimination means for protracting said marking signal or said spacing signal during the reception of impulses of said third frequency.

2. In a telegraph system, a telegraph transmitter, control means connected thereto, said means comprising a telegraph signal sending relay having an armature and marking and spacing contacts thereon, a reactance tube, a potentiometer, a grid in said tube connected to a point on said potentiometer, means interconnecting said contacts and said armature to said potentiometer, responsive to telegraph signals received by said relay, for impressing potential of a first, second and third magnitude on said grid, While said relay is in engagement with said marking contact and said spacing contact and While said armature is in transit between said contacts, respectively, for generating signals of three different frequencies for marking, spacing and transition impulses respectively, a telegraph receiver, a filter in said receiver for suppressing current having a frequency corresponding to said transition impulse, a telegraph receiving relay in said receiver, and a local locking circuit connected to said relay in said receiver for effectively adding said transition impulses to said marking and spacing impulses to eliminate bias.

3. In a telegraph signal transmitting circuit, a telegraph signal transmitting relay, a winding on said relay connected to an incoming telegraph circuit, a marking contact and a spacing contact on said relay, said spacing contact connected directly to ground, an armature on said relay engaging said contacts under control of said winding, a reactance tube, a grid in said tube, a first circuit extending from grounded positive battery directly through resistances R1, Rz, R3 and R4, sequentially, in series to ground, a second circuit extending directly from the junction between said resistances R1 and R2 to said armature, a third circuit extending directly from the junction between said resistances R2 and R3 to said marking contact, a fourth circuit extending directly from the junction between said resistances R3 and R4 to said grid, said circuits cooperating to control said tube so as to generate therein currents of three diierent frequencies, namely, f1, f2 and fs, as said armature engages said marking contact and said spacing contact and as said armature is in transit in both directions between said contacts, respectively ANDREW L. MATTE.

REFERENCES CITED TheA following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,291,369 Boughtwood July 28, 1942 1,937,131 Schroter Nov. 28, 1933 1,795,393 Herman Mar. 10, 1931 1,937,640 Clarke Dec. 5, 1933 2,045,735 Taylor June 30, 1936 

